Implantable cardioverter defibrillators (ICDs) are configured to evaluate intracardiac electrogram (EGM) signals for detecting cardiac arrhythmias. Typically a fast rhythm is detected based on EGM sensed event intervals, e.g. RR intervals. However, not all fast heart rates meeting arrhythmia detection criteria are lethal arrhythmias that require a cardioversion or defibrillation shock. For example, a fast ventricular rate may be sinus tachycardia in response to physical exertion. In other cases, oversensing of cardiac events may result in an overestimate of the actual heart rate, which can cause false tachycardia detections. While it is desirable to quickly detect and treat potentially lethal arrhythmias, such as ventricular fibrillation, it is also desirable to avoid painful defibrillation shocks when such shocks are unnecessary. A need remains for improved methods for detecting arrhythmias and discriminating between potentially lethal arrhythmias associated with hemodynamic collapse and non-lethal, fast rhythms that do not require immediate therapy.